The present invention relates to a new family of anion and cation exhange membranes, methods of producing such membrances, and their uses.
Uses of Ion Exchange Membranes
Anion exchange membranes having anion exchange groups such as quaternary ammonium groups or cation exchange membranes having cation exchange groups such as sulfonic acid groups or carboxylic acid groups have a wide variety of applications including desalination, precious metal recovery, etc. Such ion exchange membranes are critical components of advanced separation systems that may be used in:                electrodialytic concentration or desalination of electrolyte solutions,        separation of specific ions from mixture of ionic solutions,        processes such as chlor-alkali production, where the membranes are used as separators for electrolysis,        recovery of acids or alkalis through ion exchange membrane processes,        concentration of seawater to produce sodium chloride,        deminerlization of saline water, and        desalination of cheese-whey products.        
In a more specific illustration, the excessive use of fertilizers has resulted in the excessive concentration of nitrate ions in ground water. This poses a serious problem because concentrations of nitrate ions above 25 ppm are harmful to human health. Similarly, concentrations of fluoride ions significantly above the 0.5-1.5 ppm recommended level for drinking water are harmful to human health. Excessive fluoride intake may result in skeletal or dental fluorosis.
Recent studies have clearly shown a direct correlation between the intake of nitrate and blue baby syndrome (resulting in infant mortality), cancer of the womb and other problems in pregnant women. Excessive chloride has also caused panic where young and capable people have suddenly developed premature aging symptoms requiring walking sticks to move. Such health threats may be alleviated by removing excessive harmful ions from water using an anion exchange membrane in appropriate combination with a cation exchange membrane.
The use of these anion and cation exchange membranes represents an advancement over the techniques used in the past, such as reverse osmosis and ultrafiltration (Desalination, 121: 139 (1999)). More stringent regulations designed to promote public health also make the removal of these and harmful ions necessary for a wide range of industrial applications. Electrodialysis represents the most promising method to meet these regulatory requirements through the development of nitrate and fluoride ion-selective anion exchange membranes.
In fact, for treatment of a variety of industrial effluents, elctrodialysis offers significant advantages over other techniques (Pure and Applied Chemistry, 46 213-220 (1976)). One such advantage lies in the fact that wastewater constituents are neither destroyed nor chemically altered by electrodialysis. This allows the recovery of valuable products such as certain metal ions and other inorganic materials which may then have further commercial applications. Such recovery is facilitated by the fact that electrodialysis produces a concentrated, low-volume waste stream containing these products. Further, this results in high ratios of recovered water. Water retrieved through electrodialysis is relatively clean and may be used or reused with little or no further treatment. Electrodialysis processes using ion exchange membranes also represent an advantage over the currently used reverse osmosis methods for recovery of precious metals such as gold and silver because reverese osmosis membranes are susceptible to scaling and salt deposition, known as membrane fouling. Electrodialysis membranes can easily be washed with dilute acidic solutions requiring minimum maintainance and also minimum waste is generated during the process.
Another advantage lies in the properties of the membranes themselves. Electrodialysis membranes may be synthesized to be highly specific, allowing the separation of targeted ions in the electrodialysis process. Ion exchange membranes are also capable of withstanding highly acidic solutions, unlike membranes and materials used in other filtration processes. If they are durable and specific, ion exchange membranes may be cost effective even in small-scale treatment. However, these membranes remain most useful in removing salts from water, seawater, brackish water, including valuable metals from waste industrial sources because utilizing such membranes with large contact areas further reduces the cost of the process.
Current Techniques for Producing Ion Exchange Membranes
Despite the great potential of ion exchange membrane electrodialysis applications, current techniques for producing such membranes are inadequate or overly expensive. Most of the previously developed ion exchange membranes may be classified as either homogenous or heterogenous. Heterogenous membranes are prepared by incorporating the ion exchange groups into the film-forming resins by (i) dry molding or calendering mixtures of the ion exchange and film-forming materials, (ii) dispersing the ion exchange material in a solution of the film-forming polymer and then casting films from the solution and evaporating the solvent, and (iii) dispersing the ion exchange material in a partially polymerized film-forming polymer, casting films, and completing the polymerization. FIG. 1 illustrates a current heterogenouse membrane as described in U.S. Pat. No. 6,103,078 of Hitchems et al.
Homogenous ion exchange membranes are better suited because the fixed ion charges are distributed homogenously over the entire polymer matrix. In the past, a few homogenous ion exchange membranes have been prepared by (i) polymerization of mixtures of reactants that can undergo condensation polymerization (at least one of the reactants must contain a group that can be made anionic or cationic) or (ii) chain polymerization of mixtures of reactants (e.g. styrene, vinyl pyridine or divinylbenzene) that can polymerize (at least one of the reactants must contain an anionic or cationic moiety). There is a current need for the development of ion exchange membranes possessing a combination of both good electrochemical performance and high mechanical strength that can be provided by the homogenous membranes.
More specifically, previously developed heterogenous ion exchange membranes are prepared by dispersing finely divided pulverized particles of ion exchange materials in a polymeric binder. These membranes suffer from numerous disadvantages arising from their macro-sized, non-uniform particle structure. Furthermore, it is extremely difficult to overcome this problem by uniformly dispersing the ion exchange materials in the polymer binder. Because of the persistence of this problem, membranes prepared in this manner will contain zones with higher or lower concentration of the exchangeable ionic groups, leading to inequalities in ion transport. This in turn results in interuptions in ionic conductance from one side of the membrane to the other when employed in electrodialysis.
One type of heterogenous membrane, the composite membrane, attempts to overcome these problems by using thin adhesive layers for lamination of the cation or anion exchange membranes. See FIG. 1. Although this type of membrane is quite popular and exhibits tolerable stability in some processes, it is entirely unsatisfactory in harsh environments such as those with high temperatures and strong oxidizing conditions. In these harsh environments, the thin adhesive layer used to laminate the membrane is not stable and will peel off after extened use, destroying the membrane.
Homogenous membranes have generally been prepared by casting homogenous solutions of a film-forming polymer or copolymer (e.g. polystyrene-butadiene) and a polyelectrolyte (e.g. N-methyl-4-vinyl pyridinium chloride polymer). Although generally better than heterogenous membranes, even these membranes are unsatisfactory for a wide variety of applications because the mixed polyelectrolyte is not bound to the film-forming polymer. As a result, the polyelectrolyte is slowly extracted from the membrane during use, eventually resulting in a loss of conductivity as well as ion exchange capacity.
Some of the more successful ion exchange membranes have employed a wide range of support materials. Specifically, polyester (Dacron®), polyamides (Nylon®), acrylics (Orlon®), modacrylics (Dynel® or Kanecaron®), vinylidene chlorides (Saran®), tetrafluoroethylene (Teflon®), glass (Fiberglas®), rayons, polyvinyl chloride (Teviron cloth), polypropylene and the like having a visibly open structure as found in woven type fabrics have been used.
Radiation-induced grafting methods previously described and widely investigated for the preparation of ion exchange membranes have met with limited success. When practiced on a large scale, however, the levels of electrical power required to produce these membranes is economically or physically impracticable (J. Applied Polymer Sci., 76: 220-227 (2000), J. Electrochemical Society, 142: 3659-3663 (1995), J. Applied Polymer Sci., 64: 1469-1475 (1997)). To date, most of the commercially available ion exchange membranes are prepared using the copolymers of styrene (St), chloromethylstyrene and divinylbenzene (DVB), which are subsequently modified by the addition of ion exchange moieties (Reactive & Functional Polymers, 46: 39-47 (2000)). Since St-DVB copolymer is not easily molded into a membrane of sufficient mechanical strength for most applications, St, DVB and other monomers are often alternatively coated onto a support membrane made of polypropylene (PP) or polyvinyl chloride (PVC). Apart from styrene, polysulfones, polyether sulfones or polyvinylpyridine have also been used as a matrix polymer. However, these materials are all difficult to shape into thin, flexible layers most useful as ion exchange membranes. In addition, very toxic chemicals such as chloromethyl ethers (a carcinogenic compound) must be used to produce these polymers.
Anion exchangers (e.g. DD 301 541) (J. Membrane Science, 179: 101-117 (2000)) containing alkylidene epoxides bound on a polyvinyl alcohol base and prepared from the reaction of epichlorohydrin with the secondary or teriary amines have also been produced. These exchangers have the drawback that they can only be produced as balls and not as membranes. Additionally, their chemical stabiliity against acids and alkalis is unsatisfactory due to the production of polyhydroxylated polymers.
Presently, most successful anion exchange membranes have been prepared by copolymerizing chloromethylstyrene or vinyl pyridine in the presence of divinyl benzene using benzoyl peroxide as an initiator (J. Membrane Science, Part B, 37: 1773-1785 (1999)). Membranes have also been prepared by copolymerization of styrene and divinyl benzene or butadiene or copolymerization of acrylonitrile and butadiene. The ionic charges on the membranes were introduced by using quaternary salts like trimethylamine in the case of chloromethylstyrene and methyl iodide or, rarely, methylbromide in the case of vinyl pyridine monomer. These membranes have shown low electrical resistivity and good mechanical strength in addition to satisfactory transport properties i.e. transport numbers exceeding 98% for the corresponding anions such as bromide, nitrate, chloride, fluoride, sulfate, etc.
Even though these membranes have reached commercial applications in electrodialysis, they leave ample room for further improvement. The primary problem with such membranes lies in the method of preparation. The method used to prepare chloromethylstyrene is very hazardous because it necessitates the use of toxic compounds, such as chloromethylether, a known carcinogen which may even become airborn during the preparation process. Moreover, chloromethylstyrene is usually not available in large quantities. Additionally, the use of divinyl benzene makes the preparation process very costly further leading to the increased overall process cost.
In other efforts, polysulfone and polyethersulfone membranes have been prepared (J. Membrane Science, 156 61-65 (1999)). These types of membranes show increased mechanical strength and increased facility in the fabrication process. However, membrane synthesis nevertheless requires the use of hazardous chloromethylether. Moreover, these membranes exhibit transport numbers only in the range of 77% to 87% (J. Membrane Science, Part B, 37: 1773-1785 (1999)).
A few attempts have been made to synthesize anion exchange membranes by copolymerizing 4-vinylpyridine with divinyl benzene (Polymer, 40: 7243-7249 (1999)). Radiation grafting of 4-vinylpyridine onto polyethylene thin film as a support have also been attempted (J. Polymer Science, Part B: 27: 2229-2241 (2000); J. Applied Polymer Science, 64: 1469-1475 (1997)). The electrochemical properties of these membranes are poor. U.S. Pat. No. 5,936,004 was issued for the preparation of blend membranes of epichlorohydrin and polyacrylonitrile in the presence of 1,4-diazobicyclo-(2,2,2)-octane in DMF as a quaternizing agent. See the chemical structure given in FIG. 2 for an illustration of this membrane. This method avoids the use of toxic chloromethylether and the membranes appear to have better transport properties than previously reported, but they have not been commercially accepted for a variety of reasons. First, the membranes are formed solely from the polymer itself and lack any additional support. Thus their mechanical stability is impaired, especially at larger sizes. Second, the membranes are formed by starting with halogenated polyethers to produce polyethylene-based membranes. Polyethylene is not as strong as many other fibers and this contributes to further deficiencies in mechanical strength. Therefore, the membranes of U.S. Pat. No. 5,936,004 are limited in size and must be replaced frequently because of their poor mechanical stability.
Additionally, such membranes are often unable to process highly concentrated ionic solutions. Therefore, a pretreating process must be used to remove some unwanted ions prior to treatment by electrodialysis. In the case of desalination, this requirement is set forth previously (Polymer News, 25: 80-86 (2000)), which describes a desalination technique using state-of-the-art membranes. Because of the inherent limitations of these membranes, and the entire pretreatment system is required prior to electrodialysis, making the process costlier and more cumbersome.
Therefore, a need exists for the development of ion exchange membranes with good electrochemical properties coupled with excellent chemical, mechanical and thermal stability that may be prepared with minimal usage of hazardous chemicals and that may be used in a wide variety of applications and particularly in commercial applications or for concentrated ionic solutions.